We represent P as number of pigs, C as cows and H as horse
We know that 9=P+C.......1
10=C+H..........................2
6=P+C+H........................3
1+2= P+C+C+H = P+2C+H=10+9=19.......4
4-3= (P+2C+H) - (P+C+H)= C = 19-6 = 7
There will be 7 cows but it is not possible. I think there is something wrong with the question.
